This invention relates to a method and apparatus for providing access to mainframe system synchronous communications facilities via traditional asynchronous facilities while retaining the major advantages associated with synchronous facilities.
The widespread use and increasing acceptance of personal computers (PC) in both the home and office environments has created a need for communication with other computers. Personal computers are increasingly being used in businesses of all sizes to maintain data bases representing all aspects of the operation of these businesses. Personal computers are also being utilized to access large computerized data bases containing specialized information. Data transmitted over the communication links between the computerized data base and the personal computer is characterized as either synchronous or asynchronous. In a synchronous data transmission, character timing is based on synchronizing characters at the beginning of each message or block of data. In an asynchronous data transmission, each character is preceded by a start bit and followed by a stop bit to synchronize a receiver with a transmitter for receipt of each character.
In an asynchronous data communications link serial data is transmitted in a nonclocked fashion. The communications link is kept in a known state such as a mark idle which consists of a succession of binary l's. After a character is sent, the data line returns to a mark idle waiting for the next character. The start and stop bits added to each character are used to establish character and bit synchronization for the receiver equipment. The asynchronous character varies in length from five to eight bits depending upon the code used and whether a parity bit is added.
In a synchronous data communications link serial data is transmitted in a clocked fashion to establish bit by bit synchronization. The idle mode for a synchronous data link is determined by the individual system and can be either mark idling or the repetitive transmission of a special character. Information is sent in blocks or frames which typically contain many characters. Preceding these blocks of information are one or more synchronization characters used to provide block or frame synchronization.
If a synchronous modem is used in synchronous transmission, the transmitting side of the modem supplies a clock to the data terminal equipment (DTE) to be used by it to synchronously transmit data. The receiver side of the modem supplies the required synchronous clock along with the data stream to the DTE.
Synchronous transmissions provides for increased efficiency of the communications channel because start and stop bits are eliminated for each character. Synchronous operation is desirable when the data stream is steady; asynchronous operation is appropriate when the data stream is irregular.
Asynchronous communication channel hardware is more prevalent and less expensive than synchronous hardware. In addition, over switched dial lines, the speed of asynchronous modems has caught up with that of synchronous modems and asynchronous modems also provide automatic dialing.
Communication between two asynchronous ports over an asynchronous communications channel and communication between two synchronous ports over a synchronous communications channel is well known in the art.
FIG. 1 shows a typical Synchronous Data Communication System with two synchronous data ports communicating over a synchronous communications channel via a pair of synchronous modems. A brief description can be found in U.S. Pat. No. 4,569,062. That patent specifically teaches the concept of utilizing a synchronous communications channel with asynchronous data terminal equipment which requires 8 bit/byte and idle and break codes. However, there is a need for an inexpensive, reliable means of communications between synchronous data terminal equipment over an asynchronous communications channel without any regard to byte size, synchronization characters, synchronous communication protocols, break, or idle codes.